Multi-gate device with air gap spacer and fabrication methods thereof

ABSTRACT

A semiconductor device includes a substrate, nanostructures vertically suspended above the substrate, a metal gate structure wrapping each of the nanostructures, an epitaxial feature having a first sidewall in physical contact with end portions of the nanostructures, and an air gap disposed between the epitaxial feature and the metal gate structure. The air gap exposes the first sidewall of the epitaxial feature and the end portions of the nanostructures.

PRIORITY

This is a continuation of U.S. patent application Ser. No. 17/515,674,filed on Nov. 1, 2021, which is a continuation of U.S. patentapplication Ser. No. 16/582,694, filed on Sep. 25, 2019, now issued U.S.Pat. No. 11,177,344, each of which is incorporated herein by referencein its entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofprocessing and manufacturing ICs.

For example, multi-gate devices have been introduced in an effort toimprove gate control by increasing gate-channel coupling, reduceOFF-state current, and reduce short-channel effects (SCEs). One suchmulti-gate device that has been introduced is the gate-all aroundtransistor (GAA). The GAA device gets its name from the gate structurewhich can extend around the channel region providing access to thechannel on two or four sides. GAA devices are compatible withconventional complementary metal-oxide-semiconductor (CMOS) processesand their structure allows them to be aggressively scaled whilemaintaining gate control and mitigating SCEs. In conventional processes,GAA devices provide a channel in a silicon nanowire. However,integration of fabrication of the GAA features around the nanowire canbe challenging. For example, while the current methods have beensatisfactory in many respects, challenges with respect to performance ofthe resulting device may not be satisfactory in all respects.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIGS. 1A, 1B, and 1C illustrate a flowchart of an example method formaking a semiconductor device in accordance with some embodiments of thepresent disclosure.

FIGS. 2, 3, 4, 5, 6, 7 are fragmentary cross-sectional views of anexample semiconductor device in accordance with some embodiments of thepresent disclosure.

FIG. 8 illustrates a three-dimensional perspective view of an examplesemiconductor device in accordance with some embodiments of the presentdisclosure.

FIGS. 9A, 10A, 11A, 12A, 13A, 14A, 15A, 16A, 17A, 18A, 19A, 20A, 21A,22A, 23A, 24A, 25A, 26A, and 27A are corresponding fragmentarycross-sectional views of a semiconductor device in FIG. 8 taken along afirst cut A-A at intermediate stages of an embodiment of the method ofFIGS. 1A, 1B, and 1C in accordance with some embodiments of the presentdisclosure.

FIGS. 9B, 10B, 11B, 12B, 13B, 14B, 15B, 16B, 17B, 18B, 19B, 20B, 21B,22B, 23B, 24B, 25B, 26B, and 27B are corresponding fragmentarycross-sectional views of a semiconductor device in FIG. 8 taken along asecond cut B-B at intermediate stages of an embodiment of the method ofFIGS. 1A, 1B, and 1C in accordance with some embodiments of the presentdisclosure.

FIGS. 9C, 10C, 11C, 12C, 13C, 14C, 15C, 16C, 17C, 18C, 19C, 20C, 21C,22C, 23C, 24C, 25C, 26C, and 27C are corresponding fragmentarycross-sectional views of a semiconductor device in FIG. 8 taken along athird cut C-C at intermediate stages of an embodiment of the method ofFIGS. 1A, 1B, and 1C in accordance with some embodiments of the presentdisclosure.

FIGS. 9D, 10D, 11D, 12D, 13D, 14D, 15D, 16D, 17D, 18D, 19D, 20D, 21D,22D, 23D, 24D, 25D, 26D, and 27D are corresponding fragmentarycross-sectional views of a semiconductor device in FIG. 8 taken along afourth cut D-D at intermediate stages of an embodiment of the method ofFIGS. 1A, 1B, and 1C in accordance with some embodiments of the presentdisclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the disclosure.Specific examples of components and arrangements are described below tosimplify the present disclosure. These are, of course, merely examplesand are not intended to be limiting. For example, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

In addition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a feature on, connected to, and/or coupled toanother feature in the present disclosure that follows may includeembodiments in which the features are formed in direct contact, and mayalso include embodiments in which additional features may be formedinterposing the features, such that the features may not be in directcontact. In addition, spatially relative terms, for example, “lower,”“upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,”“up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for easeof the present disclosure of one features relationship to anotherfeature. The spatially relative terms are intended to cover differentorientations of the device including the features. Still further, when anumber or a range of numbers is described with “about,” “approximate,”and the like, the term is intended to encompass numbers that are withina reasonable range including the number described, such as within +/−10%of the number described or other values as understood by person skilledin the art. For example, the term “about 5 nm” encompasses the dimensionrange from 4.5 nm to 5.5 nm.

The present disclosure is generally related to semiconductor devices andthe fabrication thereof, and more particularly to multi-gatetransistors. Multi-gate transistors include those transistors whose gatestructures are formed on at least two-sides of a channel region. Thesemulti-gate devices may include a p-type metal-oxide-semiconductor deviceor an n-type metal-oxide-semiconductor multi-gate device. Specificexamples may be presented and referred to herein as FINFET, on accountof their fin-like structure. Also presented herein are embodiments of atype of multi-gate transistor referred to as a gate-all-around (GAA)device. A GAA device includes any device that has its gate structure, orportion thereof, formed on 4-sides of a channel region (e.g.,surrounding a portion of a channel region). Devices presented hereinalso include embodiments that have channel regions disposed in nanowirechannel(s), bar-shaped channel(s), and/or other suitable channelconfiguration. Presented herein are embodiments of devices that may haveone or more channel regions (e.g., nanowires) associated with a single,contiguous gate structure. However, one of ordinary skill wouldrecognize that the teaching can apply to a single channel (e.g., singlenanowire) or any number of channels. One of ordinary skill may recognizeother examples of semiconductor devices that may benefit from aspects ofthe present disclosure.

With transistor dimensions are continually scaled down to sub-10 nmtechnology nodes and below, it is generally desired to reduce straycapacitance among features of multi-gate transistors, such ascapacitance between a gate structure and source/drain contacts, in orderto increase switching speed, decrease switching power consumption,and/or decrease coupling noise of the transistors. Certain low-kmaterials, with a dielectric constant lower than that of silicon oxide,have been suggested as insulator materials, such as inner spacersinterposing between gate structures and epitaxial source/drain (S/D)features, for providing lower relative permittivity to reduce straycapacitance. However, as semiconductor technology progresses to smallergeometries, the distances between the gate structure and source/draincontacts are further reduced, resulting in still large straycapacitance. Consequently, stray capacitance in multi-gate transistorshas become more problematic. The present disclosure providesimprovements in forming gate structures that do not directly contact orabut epitaxial S/D features. Further, provided are methods of formingepitaxial S/D features with confined lateral thickness, which results infurther reduction of stray capacitance between epitaxial S/D featuresand gate structures. In some embodiments of the present disclosure, thisprovides a benefit of reducing the C_(gd) (gate-to-drain capacitance)and C_(gs) (gate-to-source capacitance) of the device. In someembodiments of the present disclosure, this provides a benefit ofreducing the C_(eff) (effective capacitance) of the device withoutobvious penalty to the I_(eff) (effective current). Therefore, the speedof the circuit could be further improved.

FIGS. 1A-C illustrate a flow chart of a method 100 for formingmulti-gate devices according to various aspects of the presentdisclosure. As used herein, the term “multi-gate device” is used todescribe a device (e.g., a semiconductor transistor) that has at leastsome gate material disposed on multiple sides of at least one channel ofthe device. In some examples, the multi-gate device may be referred toas a GAA device having gate material disposed on four sides of at leastone channel member of the device. The channel member may be referred toas “wire” or “semiconductor wire,” which is used herein to designate anymaterial portion with nanoscale, or even microscale dimensions, andhaving an elongate shape, regardless of the cross-sectional shape ofthis portion. Thus, this term designates both circular and substantiallycircular cross-section elongate material portions, and beam orbar-shaped material portions including for example a cylindrical inshape (e.g., nanowires) or substantially rectangular cross-section(e.g., nanosheets).

FIGS. 1A-C will be described below in conjunction with FIGS. 2-27D.FIGS. 2, 3, 4, 5, 6, 7 are fragmentary cross-sectional views of asemiconductor device 200 (or device 200) at various stages offabrication according to method 100. FIG. 8 is a diagrammaticperspective view of a semiconductor device 200 at a certain stage offabrication according to method 100. FIGS. 9A, 10A, 11A, 12A, 13A, 14A,15A, 16A, 17A, 18A, 19A, 20A, 21A, 22A, 23A, 24A, 25A, 26A, and 27A arecorresponding fragmentary cross-sectional views of a semiconductordevice 200 along a first cut A-A; FIGS. 9B, 10B, 11B, 12B, 13B, 14B,15B, 16B, 17B, 18B, 19B, 20B, 21B, 22B, 23B, 24B, 25B, 26B, and 27B arecorresponding fragmentary cross-sectional views of a semiconductordevice 200 along a second cut B-B being in a channel region; FIGS. 9C,10C, 11C, 12C, 13C, 14C, 15C, 16C, 17C, 18C, 19C, 20C, 21C, 22C, 23C,24C, 25C, 26C, and 27C are corresponding fragmentary cross-sectionalviews of a semiconductor device 200 along a third cut C-C being throughgate sidewall spacer features; and FIGS. 9D, 10D, 11D, 12D, 13D, 14D,15D, 16D, 17D, 18D, 19D, 20D, 21D, 22D, 23D, 24D, 25D, 26D, and 27D arecorresponding fragmentary cross-sectional views of a semiconductordevice 200 along a fourth cut D-D being in an S/D region. The method 100is merely an example, and is not intended to limit the presentdisclosure beyond what is explicitly recited in the claims. Additionalsteps can be provided before, during, and after method 100, and some ofthe steps described can be moved, replaced, or eliminated for additionalembodiments of method 100. Additional features can be added in thesemiconductor device depicted in FIGS. 2-27D and some of the featuresdescribed below can be replaced, modified, or eliminated in otherembodiments of the semiconductor device.

As with the other method embodiments and exemplary devices discussedherein, it is understood that parts of the semiconductor device 200 maybe fabricated by a CMOS technology process flow, and thus some processesare only briefly described herein. Further, the exemplary semiconductordevices may include various other devices and features, such as othertypes of devices such as additional transistors, bipolar junctiontransistors, resistors, capacitors, inductors, diodes, fuses, staticrandom access memory (SRAM) and/or other logic circuits, etc., but issimplified for a better understanding of the inventive concepts of thepresent disclosure. In some embodiments, the exemplary devices include aplurality of semiconductor devices (e.g., transistors), including PFETs,NFETs, etc., which may be interconnected. Moreover, it is noted that theprocess steps of method 100, including any descriptions given withreference to FIGS. 2-27D, as with the remainder of the method andexemplary figures provided in this disclosure, are merely exemplary andare not intended to be limiting beyond what is specifically recited inthe claims that follow.

At operation 102, the method 100 (FIG. 1A) provides a substrate 202, asshown in FIG. 2 . In some embodiments, the substrate 202 may be asemiconductor substrate such as a silicon substrate. The substrate 202may include various layers, including conductive or insulating layersformed on a semiconductor substrate. The substrate 202 may includevarious doping configurations depending on design requirements as isknown in the art. For example, different doping profiles (e.g., n wells,p wells) may be formed on the substrate 202 in regions designed fordifferent device types (e.g., n-type field effect transistors (NFET),p-type field effect transistors (PFET)). The suitable doping may includeion implantation of dopants and/or diffusion processes. The substrate202 typically has isolation features (e.g., shallow trench isolation(STI) features) interposing the regions providing different devicetypes. The substrate 202 may also include other semiconductors such asgermanium, silicon carbide (SiC), silicon germanium (SiGe), or diamond.Alternatively, the substrate 202 may include a compound semiconductorand/or an alloy semiconductor. Further, the substrate 202 may optionallyinclude an epitaxial layer (epi-layer), may be strained for performanceenhancement, may include a silicon-on-insulator (SOI) structure, and/orhave other suitable enhancement features. In an embodiment of the method100, at operation 102, an anti-punch through (APT) implant is performed.The APT implant may be performed in a region underlying the channelregion of a device for example, to prevent punch-through or unwanteddiffusion.

Still referring to FIG. 2 , at operation 104, the method 100 (FIG. 1A)forms one or more epitaxial layers over the substrate 202. In someembodiments, an epitaxial stack 204 is formed over the substrate 202.The epitaxial stack 204 includes epitaxial layers 206 of a firstcomposition interposed by epitaxial layers 208 of a second composition.The first and second composition can be different. In an embodiment, theepitaxial layers 206 are silicon germanium (SiGe) and the epitaxiallayers 208 are silicon (Si). However, other embodiments are possibleincluding those that provide for a first composition and a secondcomposition having different oxidation rates and/or etch selectivity. Insome embodiments, the epitaxial layer 206 includes SiGe and where theepitaxial layer 208 includes Si, the Si oxidation rate of the epitaxiallayer 208 is less than the SiGe oxidation rate of the epitaxial layer206. It is noted that four (4) layers of each of epitaxial layers 206and 208 are illustrated in FIG. 2 , which is for illustrative purposesonly and not intended to be limiting beyond what is specifically recitedin the claims. It can be appreciated that any number of epitaxial layerscan be formed in the epitaxial stack 204; the number of layers dependingon the desired number of channels regions for the semiconductor device200. In some embodiments, the number of epitaxial layers 208 is between2 and 10.

In some embodiments, the epitaxial layer 206 has a thickness range ofabout 2-6 nanometers (nm). The epitaxial layers 206 may be substantiallyuniform in thickness. In some embodiments, the epitaxial layer 208 has athickness range of about 6-12 nm. In some embodiments, the epitaxiallayers 208 of the stack are substantially uniform in thickness. Asdescribed in more detail below, the epitaxial layer 208 may serve aschannel region(s) for a subsequently-formed multi-gate device and itsthickness chosen based on device performance considerations. Theepitaxial layer 206 may serve to define a gap distance between adjacentchannel region(s) for a subsequently-formed multi-gate device and itsthickness chosen based on device performance considerations.

By way of example, epitaxial growth of the layers of the epitaxial stack204 may be performed by a molecular beam epitaxy (MBE) process, ametalorganic chemical vapor deposition (MOCVD) process, and/or othersuitable epitaxial growth processes. In some embodiments, theepitaxially grown layers such as, the layers 208 include the samematerial as the substrate 202. In some embodiments, the epitaxiallygrown layers 206 and 208 include a different material than the substrate202. As stated above, in at least some examples, the epitaxial layer 206includes an epitaxially grown SiGe layer and the epitaxial layer 208includes an epitaxially grown Si layer. Alternatively, in someembodiments, either of the epitaxial layers 206 and 208 may includeother materials such as germanium, a compound semiconductor such assilicon carbide, gallium arsenide, gallium phosphide, indium phosphide,indium arsenide, and/or indium antimonide, an alloy semiconductor suchas SiGe, GaAsP, AlInAs, AlGaAs, InGaAs, GaInP, and/or GaInAsP, orcombinations thereof. As discussed, the materials of the epitaxiallayers 206 and 208 may be chosen based on providing differing oxidation,etch selectivity properties. In various embodiments, the epitaxiallayers 206 and 208 are substantially dopant-free (i.e., having anextrinsic dopant concentration from about 0 cm⁻³ to about 1×10¹⁷ cm⁻³),where for example, no intentional doping is performed during theepitaxial growth process.

At operation 106, the method 100 (FIG. 1A) patterns the epitaxial stack204 to form device fins 210 (also referred to as semiconductor fins).With reference to the example of FIG. 3 , in an embodiment of operation106, a plurality of device fins 210 extending from the substrate 202 areformed. In various embodiments, each of the device fins 210 includes asubstrate portion formed from the substrate 202, portions of each of theepitaxial layers of the epitaxial stack including epitaxial layers 206and 208. In some embodiments, operation 106 includes forming a masklayer 212 over the epitaxial stack 204. The mask layer 212 includes afirst mask layer 212A and a second mask layer 212B. The first mask layer212A is a pad oxide layer made of a silicon oxide, which can be formedby a thermal oxidation. The second mask layer 212B is made of a siliconnitride (SiN), which is formed by chemical vapor deposition (CVD),including low pressure CVD (LPCVD) and plasma enhanced CVD (PECVD),physical vapor deposition (PVD), atomic layer deposition (ALD), or othersuitable process. The mask layer 212 is patterned into a mask pattern byusing patterning operations including photo-lithography and etching.

Operation 106 subsequently patterns the epitaxial stack 204 in anetching process, such as a dry etch (e.g., reactive ion etching), a wetetch, and/or other suitable process, through openings defined in thepatterned mask layer 212. The stacked epitaxial layers 206 and 208 arethereby patterned into device fins 210 with trenches 214 betweenadjacent device fins 210. Each of the device fins 210 protrudes upwardlyin the z-direction from the substrate 202 and extends lengthwise in they-direction. In various other embodiments, the device fins 210 may bepatterned by any suitable method. For example, the device fins 210 maybe patterned using one or more photolithography processes, includingdouble-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a sacrificial layer is formed over a substrate and patterned using aphotolithography process. Spacers are formed alongside the patternedsacrificial layer using a self-aligned process. The sacrificial layer isthen removed, and the remaining spacers, or mandrels, may then be usedto pattern the device fins 210. In FIG. 3 , three (3) device fins 210are spaced apart along in the x-direction. But the number of the devicefins 210 is not limited to three, and may be as small as one or morethan three.

At operation 108, the method 100 (FIG. 1A) partially fills the trenches214 with a dielectric material to form an isolation feature 216, asshown in FIG. 4 . The isolation feature 216 may include one or moredielectric layers. Suitable dielectric materials for the isolationfeature 216 may include silicon oxides, silicon nitrides, siliconcarbides, FluoroSilicate Glass (FSG), low-K dielectric materials, and/orother suitable dielectric materials. The dielectric material may bedeposited by any suitable technique including thermal growth, CVD,HDP-CVD, PVD, ALD, and/or spin-on techniques. In one embodiment, theisolation feature 216 is deposited as a conformal layer, covering eachof the device fins 210. Conformal deposition techniques may be used,such as an ALD process. The width of the trenches 214 becomes narrowerafter the deposition of the isolation feature 216. As will be shownbelow, dielectric fins will be formed in these trenches.

At operation 110, the method 100 (FIG. 1A) deposits dielectric fins 218(also referred to as dummy fins or hybrid fins) in the trenches 214, asshown in FIG. 5 . Unlike the device fins 210 configured to provideactive regions, the dielectric fins 218 are inactive and not configuredto form active regions. In some embodiments, the dielectric fins 218 areprovided to adjust fin-to-fin spacing (i.e., fin pitch) to improve theuniformity of fin density, which in turn improves and provides betterstructure fidelity. The dielectric fins 218 could also help to releasefin patterning loading effect and prevent source/drain EPI bridge. Thedielectric fins 218 may include any suitable dielectric materialincluding silicon carbide nitride, silicon carbide oxynitride, and metaloxide, such as hafnium oxide, zirconium oxide, and aluminum oxide,and/or other suitable dielectric materials, and may be deposited by anysuitable deposition process including thermal growth, CVD, HDP-CVD, PVD,ALD, and/or other suitable processes. In a particular example, thedielectric fins 218 include aluminum oxide (Al₂O₃) deposited by CVD.Following the deposition, a CMP process may be performed to removeexcess dielectric material. In some embodiments, the mask layer 212 mayfunction as a CMP stop layer.

At operation 112, the method 100 (FIG. 1A) recesses the isolationfeatures 216 to form shallow trench isolation (STI) features, as shownin FIG. 6 . Any suitable etching technique may be used to recess theisolation features 216 including dry etching, wet etching, RIE, and/orother etching methods, and in an exemplary embodiment, an anisotropicdry etching is used to selectively remove the dielectric material of theisolation features 216 without etching the device fins 210 and thedielectric fins 218. The mask layer 212 may also be removed before,during, and/or after the recessing of the isolation features 216. Insome embodiments, the mask layer 212 is removed by a CMP processperformed prior to the recessing of the isolation features 216. In someembodiments, the mask layer 212 is removed by an etchant used to recessthe isolation features 216.

In the illustrated embodiment in FIG. 6 , after operation 112 thedielectric fins 218 extend upwardly from the recessed isolation features216. Referring to FIG. 7 , in another illustrated embodiment, thedielectric fins 218 extend upwardly from the substrate 202. In thisembodiment, the isolation features 216 may first be deposited as aspacer layer over sidewalls of the device fins 210 with substrate 202exposed in the trenches 214. Subsequently, the dielectric fins 218 aredeposited over sidewalls of the isolation features 216. Accordingly, thedielectric fins 218 are in contact with the substrate 202 in thisembodiment. Thereafter, the isolation features 216 are recessed suchthat its top surface is lower than both a top surface of the device fins210 and a top surface of the dielectric fins 218.

Referring to FIGS. 6 and 7 collectively, the heights H of the devicefins 210 and dielectric fins 218 above the isolation features 216 aresubstantially the same, such as about 40 nm to about 60 nm in someembodiments. Adjacent device fin 210 and dielectric fin 218 define atrench 219 in between. The trench 219 has a width S ranges from about 5nm to about 20 nm in some embodiments. A ratio of the width W1 (measuredin x-direction) of the device fin 210 over the width W2 (measured inx-direction) of the dielectric fin 218 is about 4:1 to about 7:1. If theratio is larger than 7:1, the dielectric fin 218 may be too thin forenough mechanical strength. If the ratio is smaller than 4:1, thedielectric fin 218 may be too wide which increases fin pitch andenlarges circuit area. In a particular example, W1 ranges from about 20nm to about 70 nm, W2 ranges from about 5 nm to about 10 nm.

At operation 114, the method 100 (FIG. 1A) forms a sacrificial (dummy)gate structure 220, as shown in FIG. 8 . For the sake of simplicity andclarity, only one device fin 210 and two adjacent dielectric fins 218,and one sacrificial gate structure are depicted in FIG. 8 . But thenumber of the device fins, dielectric fins, and sacrificial gatestructures is not limited. The sacrificial gate structure 220 is formedover a portion of the device fin 210 which is to be a channel region.The sacrificial gate structure 220 defines the channel region of the GAAdevice. The sacrificial gate structure 220 includes a sacrificial gatedielectric layer (not shown) and a sacrificial gate electrode layer. Thesacrificial gate structure 220 is formed by first blanket depositing thesacrificial gate dielectric layer over the device fin 210 and thedielectric fins 218 (collectively, as fins). A sacrificial gateelectrode layer is then blanket deposited on the sacrificial gatedielectric layer and over the fins, such that the fins are fullyembedded in the sacrificial gate electrode layer. The sacrificial gateelectrode layer includes silicon such as polycrystalline silicon oramorphous silicon. The thickness of the sacrificial gate dielectriclayer is in a range from about 1 nm to about 5 nm in some embodiments.The thickness of the sacrificial gate electrode layer is in a range fromabout 100 nm to about 200 nm in some embodiments. In some embodiments,the sacrificial gate electrode layer is subjected to a planarizationoperation. The sacrificial gate dielectric layer and the sacrificialgate electrode layer may be deposited using CVD, including LPCVD andPECVD, PVD, ALD, or other suitable process. Subsequently, a mask layer222 is formed over the sacrificial gate electrode layer. The mask layer222 may include a pad silicon oxide layer 222A and a silicon nitridemask layer 222B. Subsequently, a patterning operation is performed onthe mask layer 222 and sacrificial gate dielectric and electrode layersare patterned into the sacrificial gate structure 220, as shown in FIG.8 . By patterning the sacrificial gate structure 220, the epitaxialstack 204 of the first and second semiconductor layers are partiallyexposed on opposite sides of the sacrificial gate structure 220, therebydefining source/drain (S/D) regions. In this disclosure, a source and adrain are interchangeably used and the structures thereof aresubstantially the same. By the operations explained with FIG. 8 , thestructure of FIGS. 9A-D can be obtained. In FIGS. 9A-D, the upperportion of the sacrificial gate structure 220 is not shown.

At operation 116, the method 100 (FIG. 1A) deposits sacrificial spacers224 in trenches 219 which are defined between adjacent device fin 210and dielectric fins 218 in the S/D region, as shown in FIG. 10A-D.Suitable dielectric materials for the sacrificial spacers 224 mayinclude silicon oxides, silicon nitrides, silicon carbides, siliconcarbide nitride, silicon oxide carbide, silicon carbide oxynitride,and/or other suitable dielectric materials. The sacrificial spacers 224are made of different materials from the dielectric fins 218 and theisolation feature 216 so that the sacrificial spacers 224 can beselectively etched. In some embodiments, forming of the sacrificialspacers 224 includes depositing suitable dielectric materials in thetrenches 219 and over top surfaces of the device fin 210 and thedielectric fins 218, and followed by an isotropic etching to removeexcess dielectric materials from top surfaces of the device fin 210 andthe dielectric fins 218, such that the sacrificial spacers 224 areformed with top surfaces substantially coplanar with the top surfaces ofthe device fin 210 and the dielectric fins 218, as shown in FIGS. 10C-D.

At operation 118, the method 100 (FIG. 1A) forms gate sidewall spacers226 on sidewalls of the sacrificial gate structure 220, as shown inFIGS. 11A-D. The gate sidewall spacers 226 also cover a portion of thetop surfaces of the device fin 210, the dielectric fins 218, and thesacrificial spacers 224. The gate sidewall spacers 226 may include adielectric material such as silicon oxide, silicon nitride, siliconcarbide, silicon oxynitride, SiCN films, silicon oxycarbide, SiOCNfilms, and/or combinations thereof. In some embodiments, the gatesidewall spacers 226 include multiple layers, such as main spacer walls,liner layers, and the like. By way of example, the gate sidewall spacers226 may be formed by depositing a dielectric material layer over thesacrificial gate structure 220 using processes such as, CVD process, asub-atmospheric CVD (SACVD) process, a flowable CVD process, an ALDprocess, a PVD process, or other suitable process. In some embodiments,the deposition of the dielectric material layer is followed by anetching-back (e.g., anisotropically) process to expose portions of thedevice fin 210 adjacent to and not covered by the sacrificial gatestructure 220 (e.g., S/D regions). The dielectric material layer mayremain on the sidewalls of the sacrificial gate structure 220 as gatesidewall spacers 226. In some embodiments, the etching-back process mayinclude a wet etch process, a dry etch process, a multiple-step etchprocess, and/or a combination thereof. The gate sidewall spacers 226 mayhave a thickness ranging from about 5 nm to about 20 nm.

At operation 120, the method 100 (FIG. 1B) recesses a portion of thedevice fin 210 to form recesses 230 in the S/D regions, as shown inFIGS. 12A-D. The stacked epitaxial layers 206 and 208 are etched down atthe S/D regions. In some embodiments, the substrate 202 is alsopartially etched (FIG. 12A). In many embodiments, operation 120 formsthe recesses 230 by a suitable etching process, such as a dry etchingprocess, a wet etching process, or an RIE process. In some embodiments,operation 120 selectively removes the device fin 210 without etching orsubstantially etching portions of the sacrificial spacers 224 formed onsidewalls of the device fin 210. The etching process at operation 120may implement a dry etching process using an etchant including abromine-containing gas (e.g., HBr and/or CHBR₃), a fluorine-containinggas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), other suitable gases, orcombinations thereof. The extent of which the device fin 210 is removedmay be controlled by adjusting the duration of the etching process. Insome embodiments, the etching process at operation 120 removes upperportions of the device fin 210 such that a bottom surface of therecesses 230 is below a top surface of the isolation features 216 andabove a bottom surface of the dielectric fins 218 (FIG. 12D).

At operation 122, the method 100 (FIG. 1B) forms inner spacers directlyunder the gate sidewall spacers 226. In some embodiments, operation 122first laterally etches the epitaxial layers 206 in the y-direction,thereby forming cavities 232, as shown in FIGS. 13A-D. The amount ofetching of the epitaxial layers 206 is in a range from about 5 nm toabout 20 nm along the y-direction in some embodiments. The epitaxiallayers 206 can be selectively etched by using a wet etchant such as, butnot limited to, ammonium hydroxide (NH₄OH), tetramethylammoniumhydroxide (TMAH), ethylenediamine pyrocatechol (EDP), or potassiumhydroxide (KOH) solutions. Alternatively, operation 122 may firstselectively oxidize lateral ends of the epitaxial layers 206 that areexposed in the recesses 230 to increase the etch selectivity between theepitaxial layers 206 and 208. In some examples, the oxidation processmay be performed by exposing the semiconductor device 200 to a wetoxidation process, a dry oxidation process, or a combination thereof.

Subsequently, operation 122 conformally forms an inner spacer materiallayer 234 on the lateral ends of the epitaxial layer 206 and on theepitaxial layers 208 in the recesses 230 and cavities 232. The innerspacer material layer 234 may include silicon oxides, silicon nitrides,silicon carbides, silicon carbide nitride, silicon oxide carbide,silicon carbide oxynitride, and/or other suitable dielectric materials.In the illustrated embodiment, the inner spacer material layer 234 andthe sacrificial spacers 224 are formed of the same material composition,such that the inner spacer material layer 234 and the sacrificialspacers 224 can be removed together in an etch process later on. Theinner spacer material layer 234 can be formed by ALD or any othersuitable method. By conformally forming the inner spacer material layer234, the size of cavity 232 is reduced or completely filled.

After the inner spacer material layer 234 is formed, an etchingoperation is performed to partially remove the inner spacer materiallayer 234, as shown in FIGS. 15A-D. By this etching, the inner spacermaterial layer 234 remains substantially within the cavity 232, becauseof a small volume of the cavity. Generally, plasma dray etching etches alayer in wide and flat areas faster than a layer in concave (e.g.,holes, grooves and/or slits) portions. Thus, the inner spacer materiallayer 234 can remain inside the cavities 232. The remained portions ofthe inner spacer material layer 234 is denoted as the inner spacers 234.

At operation 124, the method 100 (FIG. 1B) forms epitaxial S/D features240 in recesses 230, as shown in FIGS. 16A-D. In some embodiments, theepitaxial S/D features 240 include one or more layers of Si, SiP, SiC,and SiCP for an n-type FET or Si, SiGe, Ge for a p-type FET. For thep-channel FET, boron (B) may also be contained in the source/drain. Theepitaxial S/D features 240 are formed by an epitaxial growth methodusing CVD, ALD or molecular beam epitaxy (MBE). The epitaxial S/Dfeatures 240 are formed in contact with the epitaxial layers 208, andseparated from the epitaxial layers 206 by the inner spacers 234. Sincerecesses 230 in the x-direction are defined by sidewalls of thesacrificial spacer 224, the width W3 of the epitaxial S/D in thex-direction is confined by sidewalls of the sacrificial spacer 224 (FIG.16D), which is substantially equal to the width W1 (FIG. 6 ) of thedevice fin 210. By laterally restricting the dimensions of the epitaxialS/D features 240, the stray capacitance between the epitaxial S/Dfeatures 240 and the to-be-formed metal gate structure is reduced.

At operation 126, the method 100 (FIG. 1B) forms an interlayerdielectric (ILD) layer 250 over the epitaxial S/D features 240, as shownin FIGS. 17A-D. In some embodiments. The ILD layer 250 includes adielectric material, such as tetraethylorthosilicate (TEOS), un-dopedsilicate glass, or doped silicon oxide such as borophosphosilicate glass(BPSG), fused silica glass (FSG), phosphosilicate glass (PSG), borondoped silicon glass (BSG), other suitable dielectric materials, orcombinations thereof. The ILD layer 250 may include a multi-layerstructure having multiple dielectric materials and may be formed by adeposition process such as, for example, CVD, flowable CVD (FCVD),spin-on-glass (SOG), other suitable methods, or combinations thereof. Insome embodiments, forming the ILD layer 250 further includes performinga CMP process to planarize a top surface of the semiconductor device200, such that the top surfaces of the sacrificial gate structure 220are exposed.

At operation 128, the method 100 (FIG. 1B) removes the sacrificial gatestructure 220 to form a gate trench 254, as shown in FIGS. 18A-D. Thegate trench 254 exposes the device fin 210 and the dielectric fins 218in the channel region. The ILD layer 250 protects the epitaxial S/Dfeatures 240 during the removal of the sacrificial gate structure 220.The sacrificial gate structure 220 can be removed using plasma dryetching and/or wet etching. When the sacrificial gate electrode layer ispolysilicon and the ILD layer 250 is silicon oxide, a wet etchant suchas a TMAH solution can be used to selectively remove the sacrificialgate electrode layer. The sacrificial gate dielectric layer isthereafter removed using plasma dry etching and/or wet etching.

At operation 130, the method 100 (FIG. 1B) releases channel members fromthe channel region of the GAA device, as shown in FIGS. 19A-D. In theillustrated embodiment, channel members are epitaxial layers 208 in theform of semiconductor wires after operation 130. In the presentembodiment, the epitaxial layers 208 include silicon, and the epitaxiallayers 206 include silicon germanium. The plurality of epitaxial layers206 may be selectively removed. In some implementations, the selectivelyremoval process includes oxidizing the plurality of epitaxial layers 206using a suitable oxidizer, such as ozone. Thereafter, the oxidizedepitaxial layers 206 may be selectively removed from the gate trench254. To further this embodiment, the operation 130 includes a dryetching process to selectively remove the epitaxial layers 206, forexample, by applying an HCl gas at a temperature of about 500° C. toabout 700° C., or applying a gas mixture of CF₄, SF₆, and CHF₃. For thesake of simplicity and clarity, after operation 130, the epitaxiallayers 208 are denoted as semiconductor wires 208. At this point, asshown in FIG. 19B, vertically stacked semiconductor wires 208 are formedin the channel region of the GAA device.

At operation 132, the method 100 (FIG. 1B) forms metal gate structure258 in the gate trench 254 engaging the semiconductor wires 208 in thechannel region, as shown in FIGS. 20A-D. A gate dielectric layer 262 isformed wrapping each semiconductor wires 208 in the channel region, anda gate electrode layer 264 is formed on the gate dielectric layer 262.The inner spacers 234 separate the metal gate structure 258 fromcontacting the epitaxial S/D features 240.

In some embodiments, the gate dielectric layer 262 includes one or morelayers of a dielectric material, such as silicon oxide, silicon nitride,or high-k dielectric material, other suitable dielectric material,and/or combinations thereof. Examples of high-k dielectric materialinclude HfO₂, HfSiO, HfSiON, HfTaO, HfSiO, HfZrO, zirconium oxide,aluminum oxide, titanium oxide, hafnium dioxide-alumina (HfO₂—Al₂O₃)alloy, other suitable high-k dielectric materials, and/or combinationsthereof. In some embodiments, the gate dielectric layer 262 includes aninterfacial layer (not shown) formed between the channel layers and thedielectric material. The gate dielectric layer 262 may be formed by CVD,ALD or any suitable method. In one embodiment, the gate dielectric layer262 is formed using a highly conformal deposition process such as ALD inorder to ensure the formation of a gate dielectric layer having auniform thickness around each channel layers. The thickness of the gatedielectric layer 262 is in a range from about 1 nm to about 6 nm in someembodiments.

The gate electrode layer 264 is formed on the gate dielectric layer 262to surround each channel layer. The gate electrode layer 264 includesone or more layers of conductive material, such as polysilicon,aluminum, copper, titanium, tantalum, tungsten, cobalt, molybdenum,tantalum nitride, nickel silicide, cobalt silicide, TiN, WN, TiAl,TiAlN, TaCN, TaC, TaSiN, metal alloys, other suitable materials, and/orcombinations thereof. The gate electrode layer 264 may be formed by CVD,ALD, electro-plating, or other suitable method. The gate dielectriclayer 262 and the gate electrode layer 264 may also be deposited overthe upper surface of the ILD layer 250. The gate dielectric layer 262and the gate electrode layer 264 formed over the ILD layer 250 are thenplanarized by using, for example, CMP, until the top surface of the ILDlayer 250 is revealed. In some embodiments, after the planarizationoperation, the gate electrode layer 264 is recessed and a cap insulatinglayer (not shown) is formed over the recessed gate electrode 264. Thecap insulating layer includes one or more layers of a siliconnitride-based material, such as SiN. The cap insulating layer can beformed by depositing an insulating material followed by a planarizationoperation. In certain embodiments of the present disclosure, one or morework function adjustment layers (not shown) are interposed between thegate dielectric layer 262 and the gate electrode 264. The work functionadjustment layers are made of a conductive material such as a singlelayer of TiN, TaN, TaAlC, TiC, TaC, Co, Al, TiAl, HfTi, TiSi, TaSi orTiAlC, or a multilayer of two or more of these materials. For then-channel FET, one or more of TaN, TaAlC, TiN, TiC, Co, TiAl, HfTi, TiSiand TaSi is used as the work function adjustment layer, and for thep-channel FET, one or more of TiAlC, Al, TiAl, TaN, TaAlC, TiN, TiC andCo is used as the work function adjustment layer. The work functionadjustment layer may be formed by ALD, PVD, CVD, e-beam evaporation, orother suitable process. Further, the work function adjustment layer maybe formed separately for the n-channel FET and the p-channel FET whichmay use different metal layers.

At operation 134, the method 100 (FIG. 1B) forms contact holes 268 inthe ILD layer 250 by using dry etching, thereby exposing the upperportion of the epitaxial S/D features 240, as shown in FIGS. 21A-D. Thecontact holes 268 has a width W4 (in the x-direction) that is largerthan the width W3 of the epitaxial S/D features 240 (FIG. 21D). In someembodiments, the width W4 ranges from about 30 nm to about 80 nm. In theillustrated embodiment, a length L4 of the contact holes 268 (in they-direction) is smaller than a length of the epitaxial S/D features 240(in the y-direction) (FIG. 21A). The contact holes 268 also expose aportion of the top surface 270 of the sacrificial spacers 224. As to beshown later on, an etch process removes the sacrificial spacers 224 andinner spacers 234 through the exposed top surface 270. The exposed topsurface 270 has a width W5 (in the x-direction) on each side of theepitaxial S/D features 240 that is about one eighth (⅛) to about onethird (⅓) of the width W3 of the epitaxial S/D features 240. If thewidth W5 is smaller than one eighth (⅛) of the width W3, the openingwill be too small for etchants to effectively enter to remove thesacrificial spacers 224 and inner spacers 234 and also difficult foretching by-products to be removed. If the width W5 is larger than onethird (⅓) of the width W3, the opening will be too large for a later-oncapping process to seal the to-be-formed air gap. In some embodiments,the width W5 ranges from about 5 nm to about 20 nm.

At operation 136, the method 100 (FIG. 1C) performs one or moreselective etching processes to remove the sacrificial spacers 224 andsubsequently the inner spacers 234, thereby forming an air gap 280, asshown in FIGS. 22A-D. The etching is performed through the partiallyexposed top surface 270 (FIG. 21D) of the sacrificial spacers 224. Theetching process(es) may implement any suitable etchant configured toremove the sacrificial spacers 224 and the inner spacers 234 withoutremoving or substantially removing the epitaxial S/D features 240, thedielectric fins 218, the isolation features 216, and the ILD layer 250.In some examples, the etching process may be an isotropic etchingprocess (e.g., an isotropic dry etching or an isotropic wet etchingprocess) that implements an etchant that includes hydrofluoric acid(HF), ammonia (NH₃), nitrogen trifluoride (NF₃), other suitableetchants, or combinations thereof. Since the sacrificial spacers 224 isin contact with sidewalls of the inner spacers 234 (FIG. 21C), after thesacrificial spacers 224 are removed, the inner spacers 234 are exposedand thereby subsequently removed. In the illustrated embodiment, thesacrificial spacers 224 and the inner spacers 234 have the same materialcomposition and can be removed by the same etchant. In some otherembodiments, the sacrificial spacers 224 and the inner spacers 234 havedifferent material compositions and can be removed by differentetchants. Removing the sacrificial spacers 224 and the inner spacers 234releases the space occupied by the sacrificial spacers 224 and the innerspacers 234, thereby forming an air gap 280. The exposed top surface 270is denoted as opening 270 when the air gap 280 is formed.

As used herein, the term “air gap” is used to describe a void defined bysurrounding substantive features, where a void may contain air,nitrogen, ambient gases, gaseous chemicals used in previous or currentprocesses, or combinations thereof. In the illustrated embodiment, theair gap 280 is continuous. In the region between two adjacentsemiconductor wires 208, the air gap 280 fully surrounds a portion ofthe epitaxial S/D feature 240. In the region directly under the gatesidewall spacer 226, the air gap 280 fully surrounds some of thesemiconductor wires 208 (the topmost semiconductor wire 208 may haveonly three surfaces exposed to the air gap 280, as shown in FIG. 22C).Since the inner spacers 234 are removed, the gate structure 258 directlyfaces (in the y-direction) the epitaxial S/D feature 240 through the airgap 280. In the illustrated embodiment, the gate dielectric layer 262 ofthe gate structure 258 directly faces (in the y-direction) the epitaxialS/D feature 240 through the air gap 280. The two dielectric fins 218also have two opposing sidewalls directly facing (in the x-direction)each other through the air gap 280. The air gap 280 reduces the straycapacitance in the GAA device.

At operation 138, the method 100 (FIG. 1C) forms a silicide layer 284over the epitaxial S/D features 240, which also caps the air gap 280, asshown in FIGS. 23A-D. The silicide layer 284 is laterally larger (in thex-direction) than the width W3 of the epitaxial S/D features 240. Asdiscussed above, the opening 270 of the air gap 280 is configured toaccommodate the formation of a silicide layer to fully cap the air gap280. In many embodiments, the silicide layer 284 includes nickelsilicide, cobalt silicide, tungsten silicide, tantalum silicide,titanium silicide, platinum silicide, erbium silicide, palladiumsilicide, other suitable silicide, or combinations thereof. The silicidelayer 284 is formed by a suitable method. In one example, a metal layer(e.g., nickel) may be deposited over the semiconductor device 200 by adeposition process such as CVD, ALD, PVD, other suitable processes, orcombinations thereof. In the illustrated embodiment, operation 138deposits Titanium in a PVD process. Then, the semiconductor device 200is annealed to allow the metal layer and the semiconductor materials ofthe epitaxial S/D features 240 to react and form the silicide layer 284.Thereafter, the un-reacted metal layer is removed, leaving the silicidelayer 284 over the epitaxial S/D features 240. In some examples, thesilicide layer 284 is formed to have a thickness of about 2 nm to about3 nm. The silicide layer 284 may be disposed not only on the top surfaceof the epitaxial S/D features 240 but also at least on bottom surfacesof the ILD layer 250 exposed in the air gap 280, as shown in FIG. 23D.After the capping process, the air gap 280 vertically extends from theisolation feature 216 to the silicide layer 284.

At operation 140, the method 100 (FIG. 1C) forms S/D contacts 290 overthe silicide layer 284 to be in electrical contact with correspondingepitaxial S/D features 240, as shown in FIGS. 24A-D. Each S/D contact290 may include one or more conductive layers and may be formed usingany suitable methods such as ALD, CVD, PVD, plating, and/or othersuitable processes. In some embodiments, each S/D contact 290 includes aseed metal layer and a fill metal layer. In various embodiments, theseed metal layer includes cobalt (Co), tungsten (W), ruthenium (Ru),nickel (Ni), other suitable metals, or combinations thereof. The fillmetal layer may include copper (Cu), tungsten (W), aluminum (Al), cobalt(Co), other suitable materials, or combinations thereof.

The method 100 after operation 140 may perform additional processingsteps. For example, additional vertical interconnect features such asvias, horizontal interconnect features such as lines, and/or multilayerinterconnect features such as metal layers and interlayer dielectricscan be formed over the semiconductor device 200. The variousinterconnect features may implement various conductive materialsincluding copper (Cu), tungsten (W), cobalt (Co), aluminum (Al),titanium (Ti), tantalum (Ta), platinum (Pt), molybdenum (Mo), silver(Ag), gold (Au), manganese (Mn), zirconium (Zr), ruthenium (Ru), theirrespective alloys, metal silicides, other suitable materials, orcombinations thereof. The metal silicides may include nickel silicide,cobalt silicide, tungsten silicide, tantalum silicide, titaniumsilicide, platinum silicide, erbium silicide, palladium silicide, othersuitable metal silicides, or combinations thereof.

FIGS. 25A-D illustrate another embodiment of the semiconductor device200. Compared with FIGS. 24A-D, one difference is that the silicidelayer 284 does not fully cover a bottom surface of the ILD layer 250that is exposed in the air gap 280 (FIG. 25D). Therefore, the air gap280 still exposes a portion of the bottom surface of the ILD layer 250.FIGS. 26A-D illustrate yet another embodiment of the device 200. Asdiscussed above, the dielectric fins 218 may protrude from the substrate202 in some embodiments. Compared with FIGS. 24A-D, one difference isthat the dielectric fins are in contact with the substrate 202. FIGS.27A-D illustrate yet another embodiment of the semiconductor device 200.Compared with FIGS. 24A-D, one difference is that a portion of the innerspacers 234 is remained in the cavities 232 (now a portion of the airgap 280), because of a small volume of the cavity. The remaining portionof the inner spacers 234 seals the gate structure 258 from directlyexposing in the air gap 280.

Although not intended to be limiting, one or more embodiments of thepresent disclosure provide many benefits to a semiconductor device andthe formation thereof. The present disclosure provides methods offorming a GAA device with laterally confined epitaxial S/D features andair gap as inner spacer, where the air gap further surrounds theepitaxial S/D features and channel members of the GAA device.Accordingly, this provides a benefit of reducing the C_(gd)(gate-to-drain capacitance) and C_(gs) (gate-to-source capacitance) ofthe device. In some embodiments of the present disclosure, this providesa benefit of reducing the C_(eff) (effective capacitance) of the devicewithout obvious penalty to the I_(eff) (effective current). Therefore,the speed of the circuit could be further improved.

In one exemplary aspect, the present disclosure is directed to asemiconductor device. The semiconductor device includes a substrate;semiconductor wires disposed over the substrate; a gate structurewrapping around each of the semiconductor wires; and an epitaxialsource/drain (S/D) feature in contact with the semiconductor wires,wherein a portion of the epitaxial S/D feature is horizontallysurrounded by an air gap. In some embodiments, the gate structuredirectly faces the portion of the epitaxial S/D feature through the airgap. In some embodiments, the semiconductor device further includes aninner spacer feature disposed on the gate structure, wherein the innerspacer feature directly faces the portion of the epitaxial S/D featurethrough the air gap. In some embodiments, at least one of thesemiconductor wires has a portion vertically surrounded by the air gap.In some embodiments, the semiconductor device further includes a gatespacer disposed on sidewalls of the gate structure, wherein the portionof the at least one of the semiconductor wires is directly under thegate spacer. In some embodiments, the semiconductor device furtherincludes an isolation feature disposed on the substrate and a silicidelayer disposed on the epitaxial S/D feature, wherein the air gapvertically extends from the isolation feature to the silicide layer. Insome embodiments, the semiconductor wires extend lengthwise along afirst direction; the semiconductor wires have a first width along asecond direction perpendicular to the first direction; the portion ofthe epitaxial S/D feature has a second width along the second direction;and the first and second width are substantially equal. In someembodiments, the semiconductor device further includes a firstdielectric fin and a second dielectric fin, wherein the semiconductorwires interpose between the first and second dielectric fins. In someembodiments, the air gap horizontally extends from a first sidewall ofthe first dielectric fin to a second sidewall of the second dielectricfin. In some embodiments, a bottom surface of the epitaxial S/D featureis above bottom surfaces of the first and second dielectric fins. Insome embodiments, the first and second dielectric fins are in contactwith the substrate.

In another exemplary aspect, the present disclosure is directed to amulti-gate semiconductor device. The multi-gate semiconductor deviceincludes a substrate; semiconductor wires vertically stacked over eachother and above the substrate; a gate structure disposed over a channelregion of the semiconductor wires and surrounding each of thesemiconductor wires; an epitaxial source/drain (S/D) feature adjacentthe semiconductor wires; and a first dielectric fin and a seconddielectric fin disposed over the substrate, wherein the first and seconddielectric fins sandwich the semiconductor wires and the epitaxial S/Dfeature. In some embodiments, a width of the epitaxial S/D feature isnot larger than a width of the semiconductor wires. In some embodiments,the first dielectric fin has a first sidewall directly faces theepitaxial S/D feature and the second dielectric fin has a secondsidewall directly faces the epitaxial S/D feature. In some embodiments,the first sidewall of the first dielectric fin is spaced from theepitaxial S/D feature and the second sidewall of the second dielectricfin is spaced from the epitaxial S/D feature. In some embodiments, thegate structure, the epitaxial S/D feature, and the first and seconddielectric fins jointly define a void that surrounds the epitaxial S/Dfeature. In some embodiments, the multi-gate semiconductor devicefurther includes a gate spacer disposed over sidewalls of the gatestructure, wherein a portion of the semiconductor wires directly underthe gate spacer is surrounded by the void.

In another exemplary aspect, the present disclosure is directed to amethod of manufacturing a semiconductor device. The method includesforming a semiconductor fin protruding from a substrate, thesemiconductor fin having a channel region and a source/drain (S/D)region; forming first and second dielectric fins sandwiching thesemiconductor fin; forming a first gate stack over the semiconductor finin the channel region; depositing sacrificial spacers in the S/D regionand between the first and second dielectric fins; recess thesemiconductor fin in the S/D region, resulting in a trench between thesacrificial spacers; forming an S/D epitaxial layer in the trench; andremoving the sacrificial spacers, resulting in an air gap between theS/D epitaxial layer and the first dielectric fin and between the S/Depitaxial layer and the second dielectric fin. In some embodiments,where the semiconductor fin includes a stack of alternating firstsemiconductor layers and second semiconductor layers, the method furtherincludes removing the first gate stack, resulting in a gate trench;removing the second semiconductor layers from the gate trench; andforming a second gate stack wrapping the first semiconductor layers inthe gate trench. In some embodiments, the method further includesforming a silicide layer over the S/D epitaxial layer, the silicidelayer capping the air gap.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a substrate;a plurality of nanostructures vertically suspended above the substrate;a metal gate structure wrapping each of the nanostructures; an epitaxialfeature having a first sidewall in physical contact with end portions ofthe nano structures; and an air gap disposed between the epitaxialfeature and the metal gate structure, wherein the air gap exposes thefirst sidewall of the epitaxial feature and the end portions of thenanostructures.
 2. The semiconductor device of claim 1, wherein theepitaxial feature has a second sidewall and a third sidewall generallyextending along a lengthwise direction of the nanostructures, andwherein the air gap exposes the second sidewall and the third sidewallof the epitaxial feature.
 3. The semiconductor device of claim 2,wherein the epitaxial feature has a fourth sidewall opposing the firstsidewall, and wherein the air gap exposes the fourth sidewall of theepitaxial feature.
 4. The semiconductor device of claim 1, furthercomprising: a gate spacer disposed on sidewalls of the metal gatestructure, wherein a top surface of a topmost one of the nanostructuresis in physical contact with the gate spacer, and the air gap exposes abottom surface of the topmost one of the nanostructures.
 5. Thesemiconductor device of claim 4, wherein a portion of a bottommost oneof the nanostructures directly under the gate spacer is fully surroundedby the air gap.
 6. The semiconductor device of claim 1, furthercomprising: a silicide feature atop the epitaxial feature, wherein thesilicide feature has a larger width than the epitaxial feature in across section perpendicular to a lengthwise direction of thenanostructures.
 7. The semiconductor device of claim 6, furthercomprising: an interlayer dielectric layer atop the epitaxial feature,wherein the silicide feature is in physical contact with a bottomsurface of the interlayer dielectric layer.
 8. The semiconductor deviceof claim 1, further comprising: first and second dielectric featuresover the substrate, wherein the first and second dielectric featuressandwich the nanostructures and the epitaxial feature, and wherein theair gap exposes sidewalls of the first and second dielectric features.9. The semiconductor device of claim 1, further comprising: innerspacers disposed between the metal gate structure and the first sidewallof the epitaxial feature, wherein the air gap exposes the inner spacers.10. The semiconductor device of claim 1, further comprising: a fin-shapestructure directly under the nanostructures and the epitaxial feature;and an isolation structure disposed on sidewalls of the fin-shapestructure, wherein the air gap exposes a top surface of the isolationstructure.
 11. A semiconductor device, comprising: a substrate; firstand second dielectric features disposed over the substrate; a channellayer extending lengthwise in a first direction; a gate structure acrossthe channel layer; and an epitaxial feature abutting the channel layer,wherein: the first and second dielectric features sandwich and arespaced apart from the epitaxial feature, and in a first cross sectionperpendicular to the first direction, the epitaxial feature has a firstsidewall directly facing the first dielectric feature and a secondsidewall directly facing the second dielectric feature.
 12. Thesemiconductor device of claim 11, wherein: the first and seconddielectric features sandwich and are spaced apart from the channellayer, and in a second cross section perpendicular to the firstdirection, the channel layer has a first sidewall directly facing thefirst dielectric feature and a second sidewall directly facing thesecond dielectric feature.
 13. The semiconductor device of claim 11,wherein the gate structure is in physical contact with sidewalls of thefirst and second dielectric features.
 14. The semiconductor device ofclaim 11, wherein in a second cross section perpendicular to a topsurface of the substrate and along the first direction, the epitaxialfeature has a third sidewall directly facing the gate structure.
 15. Thesemiconductor device of claim 11, further comprising: inner spacersdisposed between the gate structure and the epitaxial feature, wherein:the inner spacers are spaced apart from the epitaxial feature, and in asecond cross section perpendicular to a top surface of the substrate andalong the first direction, the epitaxial feature has a third sidewalldirectly facing the inner spacers.
 16. A method of manufacturing asemiconductor device, comprising: forming an epitaxial stack over asubstrate, the epitaxial stack including interleaved first and secondsemiconductor layers, the first and second semiconductor layersincluding different material compositions; patterning the epitaxialstack to form a fin; forming a sacrificial gate stack over the fin;depositing a sacrificial layer on sidewalls of the fin; depositing agate spacer on sidewalls of the sacrificial gate stack and directlyabove the sacrificial layer; recessing a portion of the fin to form arecess; forming an epitaxial feature in the recess, the epitaxialfeature abutting the fin; depositing a dielectric layer covering theepitaxial feature; removing the sacrificial gate stack to form a trench;removing the second semiconductor layers from the trench to expose thefirst semiconductor layers; depositing a metal gate stack in the trench;patterning the dielectric layer to expose a top surface of thesacrificial layer; and etching the sacrificial layer to form an opening,the opening exposing sidewalls of the epitaxial feature.
 17. The methodof claim 16, wherein the depositing of the sacrificial layer is afterthe depositing of the sacrificial gate stack.
 18. The method of claim16, wherein the opening exposes a bottom surface of the gate spacer. 19.The method of claim 16, further comprising: laterally recessing endportions of the second semiconductor layers from the recess to formcavities; forming inner spacers in the cavities; and removing at least aportion of the inner spacers from the opening, such that the openingextends laterally between the metal gate stack and the epitaxialfeature.
 20. The method of claim 16, wherein the patterning of thedielectric layer also exposes a top surface of the epitaxial feature.